Fig 3. Effect of experimental preparation treatments on gross primary productivity (GPP).

Gross primary productivity means ± 1 standard error from nutrient diffusing substrate (NDS) field experiments. (A) Results from experiments 1–2 (total n = 103), where treatments consisted of crossing four chemicals (K₂HPO₄, KH₂PO₄, Na₂HPO₄, and NaH₂PO₄) and two heat treatments (agar and phosphate heated together vs. heated separately, denoted as “HT” and “HS”). An agar-only control treatment was included in each experiment. While no individual treatments significantly differed from the control, we found that monobasic phosphate forms significantly inhibited GPP compared to dibasic forms. (B) Results from experiment 3 (total n = 104), where treatments included the same factors as experiments 1–2, except low (0.05 M) and high (0.5 M) phosphate concentrations were included as an additional factor. Gross primary productivity values by concentration are presented by averaging over all other factors.